In recent years, there has been an increasing interest in a network that uses a small-size, low-power-consumption wireless terminal, such as a WPAN (Wireless Personal Area Network) and a sensor network. Also, as a system similar to the network, an active RF tag system that transmits a wireless signal by itself, can be mentioned.
FIG. 13 shows a conventional wireless communication system 10. In FIG. 13, the wireless communication system 10 includes a control device (AP: Access Point) 11, and first to third communication terminals (STA: Station) 12-1 to 12-3. The first to third communication terminals 12-1 to 12-3 exist within a communication area 15 of the control device 11. The control device 11 periodically broadcasts a beacon packet to the first to third communication terminals 12-1 to 12-3 which are all the communication terminals existing within the communication area 15. The beacon packet includes control information. The first to third communication terminals 12-1 to 12-3 receive the beacon packet from the control device 11, and communicate with the control device 11 based on the control information included in the beacon packet.
Here, in the wireless communication system 10, various methods can be adopted as an access method for access between the control device 11 and each of the communication terminals 12-1 to 12-3. For example, CSMA (Carrier Sense Multiple Access), TDMA (Time Division Multiple Access), FDMA (Frequency Division Multiple Access), SDMA (Space Division Multiple Access), and the like, can be mentioned as the access method.
In a network of the wireless communication system 10, a transmission speed is low (several kbps to several hundred kbps), and a distance traveled by a wireless signal is short (approximately several meters to several tens of meters). On the other hand, each of the communication terminals 12-1 to 12-3 is characterized by having a small size and low power consumption that allows the communication terminal to be driven by a battery for several years. Accordingly, in order to reduce power consumption of each of the communication terminals 12-1 to 12-3, a communication protocol and a frame format are devised. For example, a frame format including an active period in which each of the communication terminals 12-1 to 12-3 performs communication and an inactive period in which each of the communication terminals 12-1 to 12-3 does not perform communication is used. In the inactive period, each of the communication terminals 12-1 to 12-3 does not perform communication and therefore can be in a sleep mode. That is, by setting the inactive period long, a time period in which each of the communication terminals 12-1 to 12-3 is in the sleep mode becomes long, and thus the power consumption can be reduced.
FIG. 14 shows a conventional frame format 50. In FIG. 14, a frame 51 which is described in a frame format 50 includes an active period 52 and an inactive period 53.
The active period 52 is a period in which each of the communication terminals 12-1 to 12-3 performs communication, and has a plurality of time slots. The plurality of time slots of the active period 52 are shared among all the communication terminals existing within the network of the wireless communication system 10.
Among the plurality of time slots of the active period 52, the first time slot is allocated as a period for transmitting and receiving a beacon packet 54. In the first time slot, the control device 11 broadcasts the beacon packet 54 to the first to third communication terminals 12-1 to 12-3 which are all the communication terminals existing within the communication area 15. The beacon packet 54 includes control information concerning the frame 51, such as the number of time slots of the active period 52, allocation of the time slots, the length of the active period 52, the length of the inactive period 53, and a time until transmission of the next beacon packet.
Among the plurality of time slots of the active period 52, the time slots other than the first time slot are used for communication between the control device 11 and each of the communication terminals 12-1 to 12-3.
On the other hand, the inactive period 53 is a period in which each of the communication terminals 12-1 to 12-3 does not perform communication. In the inactive period 53, each of the communication terminals 12-1 to 12-3 is in a sleep mode, and thereby the power consumption is reduced.
FIG. 15 shows a communication sequence between the control device 11 and each of the communication terminals 12-1 to 12-3 in the wireless communication system 10. Here, for convenience of the description, the third communication terminal 12-3 is omitted.
In the first time slot of an active period 52-1, the control device 11 broadcasts a beacon packet 54-1 to each of the communication terminals 12-1 and 12-2. Each of the communication terminals 12-1 and 12-2 receives the beacon packet 54-1 from the control device 11, and acquires control information included in the beacon packet 54-1.
Subsequently, in the active period 52-1, the control device 11 and each of the communication terminals 12-1 and 12-2 communicate with each other. For example, each of the communication terminals 12-1 and 12-2 transmits data to the control device 11, and, in response thereto, the control device 11 transmits response data to each of the communication terminals 12-1 and 12-2.
In an inactive period 53-1, the control device 11 and each of the communication terminals 12-1 and 12-2 do not communicate with each other. In the inactive period 53-1, the control device 11 and each of the communication terminals 12-1 and 12-2 enter the sleep mode, and thereby power consumption can be reduced. Here, the length of the inactive period 53-1 is preliminarily notified by the control information included in the beacon packet 54-1.
When the inactive period 53-1 ends and a next active period 52-2 starts, the control device 11 and each of the communication terminals 12-1 and 12-2 recover from the sleep mode to an operation mode, and prepare for communication of a next frame.
In the active period 52-2, in the first time slot, the control device 11 broadcasts the beacon packet 54-2 to each of the communication terminals 12-1 and 12-2. Then, by repeating the same process described above, the control device 11 and each of the communication terminals 12-1 and 12-2 communicate with each other.
As shown in FIG. 15, in the active period 52-2, communication from the first communication terminal 12-1 to the control device 11 fails. Although data is transmitted from the first communication terminal 12-1 to the control device 11, the control device 11 cannot properly receive the data transmitted from the first communication terminal 12-1. In this case, the control device 11 cannot transmit response data to the first communication terminal 12-1. Since no response data is transmitted from the control device 11 even when a predetermined time period elapses, the first communication terminal 12-1 determines that the communication fails. Then, the first communication terminal 12-1 re-transmits data to the control device 11. Here, it is assumed that the control device 11 can properly receive the data re-transmitted from the first communication terminal 12-1. In this case, in response to the re-transmitted data, the control device 11 transmits response data to the first communication terminal 12-1, and the communication successfully completes.
FIG. 16 shows a channel band of a wireless channel. FIG. 16 shows a spectrum mask for channels CH1 to CH6, with the horizontal axis indicating a frequency. First to sixth channel bands 91 to 96 are channel bands of the channels CH1 to CH6, respectively.
The control device 11 selects a channel to be used for communication, from the channels CH1 to CH6, and broadcasts a beacon packet to the first to third communication terminals 12-1 to 12-3 which are all the communication terminals existing within the communication area 15. Each of the communication terminals 12-1 to 12-3 performs channel scanning, and participates in a network of a channel through which a beacon packet is received. In this manner, communication between the control device 11 and each of the communication terminals 12-1 to 12-3, which exist in the communication area 15 managed by the control device 11, starts.
A frequency band used by the network of the wireless communication system 10 described above is often shared with another network system. This causes a problem of interference between systems or between networks.
For example, at the 950 MHz band, a passive RF tag and an active RF tag partly use the same channel. At the 2.4 GHz band, more network systems such as a wireless LAN share a frequency band. Moreover, in a case where a system having large transmission power is used in an adjacent frequency band, for example, in a case where a mobile phone or the like is used, a wireless terminal which uses a simple, small-size radio frequency component such as an active RF tag cannot obtain channel selectivity and thus may suffer from interference.
In a frequency band in which interference from another network system is large, it is necessary to select a channel from a plurality of channels so as to avoid the interference from the another network system and to change a channel used by the communication terminal within the network.
FIG. 17 shows a situation in which the first communication terminal 12-1 changes a use channel by using the frame 51 which is described in the frame format 50 shown in FIG. 14. In FIG. 17, communication of a frame 51-1 is performed using a channel CH1. Here, it is assumed that, in the active period 52-1, the channel CH1 is largely influenced by interference from another network system. The control device 11 determines that communication with the first communication terminal 12-1 cannot be normally performed with the channel CH1, and transmits information for changing a use channel from the channel CH1 to a channel CH2 by using a beacon packet 54-2.
In the active period 52-2, the first communication terminal 12-1 receives the beacon packet 54-2 including information for changing the use channel from the channel CH1 to the channel CH2. However, in the active period 52-2, the control device 11 and the first communication terminal 12-1 communicate with each other by using the channel CH1, and after the active period 52-2 ends, in an inactive period 53-2, the use channel is changed to the channel CH2.
Then, for a next frame 51-3, the control device 11 and the first communication terminal 12-1 communicate with each other by using the channel CH2. When it is determined in the active period 52-3 that the channel CH2 is largely influenced by interference from another network system, the use channel is further changed from the channel CH2 to a channel CH3.
In this manner, in a frequency band that suffers from large interference from another network system, the control device 11 selects a channel to be used for communication from the channels CH1 to CH6 so as to avoid the interference from the another network system, and causes each communication terminal within the network to change a use channel.
Patent Literature 1 discloses a method of avoiding interference by performing communication using the channel CH2 in an inactive period, in a case where it is determined in an active period that the channel CH1 is largely influenced by interference from another network system.